Method and a system for registering a 3d pre acquired image coordinates system with a medical positioning system coordinate system and with a 2d image coordinate system

ABSTRACT

A method for registering a three dimensional (3D) coordinates system with a Medical Positioning System (MPS) coordinate system and with a two dimensional (2D) coordinate system, includes acquiring at least one 2D image of a volume of interest, the volume of interest including at least one tubular organ within the body of a patient. The 2D image is associated with the 2D coordinate system, and a plurality of MPS points is acquired, within the at least one tubular organ. The MPS points are associated with the MPS coordinate system, the MPS coordinate system being registered with the 2D coordinate system. A 3D image model is extracted of the at least one tubular organ form a pre-acquired 3D image of the volume of interest. A volumetric model of the at least one tubular organ from the 2D image is estimated and from the acquired MPS points, the 3D coordinate system is registered with the MPS coordinate system and with the 2D coordinate system by matching the extracted 3D image model and the estimated volumetric model of the at least one tubular organ.

FIELD OF THE DISCLOSED TECHNIQUE

The disclosed technique relates to medical imaging and positioningsystems in general, and in particular, to methods and systems forregistering the coordinates of a three dimensional (3D) pre-acquiredimage with a Medical Positioning System (MPS), the MPS being registeredwith a two-dimensional (2D) real-time medical image.

BACKGROUND OF THE DISCLOSED TECHNIQUE

Superimposing a real-time representation of a medical device, such as acatheter or a biopsy needle, tracked by a Medical Positioning X-ray,Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET) andthe like, during a medical procedure, is known in the art. This medicalimage serves as a map, aiding medical staff, performing a medicalprocedure, to navigate the medical device within a volume of interest ina body of a patient, subjected to this procedure. In order for thatsuperposition to reflect the true position of the medical device withinthat volume of interest, it is required to register the coordinatesystem associated with the MPS with the coordinate system associatedwith the medical image.

U.S. Pat. No. 6,149,592 to Yanof et al, entitled “IntegratedFlouroscopic Image Data, Volumetric Image Data, and Surgical DevicePosition Data” is directed to a system for integrating a CT scanner,fluoroscopic x-ray device and a mechanical arm type minimally invasivetype surgical tool. In one embodiment, mechanical interconnections,between the CT scanner and the fluoroscopic device, provide a fixed andknown offset there between. Mechanical interconnection between thesurgical tool and the CT scanner measured by resolvers and encodersprovide indication of the position and orientation of the surgical toolrelative to the CT scanner. Because the fluoroscopic system is alsomechanically constrained, the position and orientation of the surgicaltool relative to the fluoroscopic system is also known.

In another embodiment, a plurality of transmitters, such as LightEmitting Diodes (LED), are mounted in a fixed and known relationship tothe surgical tool or pointer. An array of receivers is mounted in afixed relationship to the CT scanner. The surgical tool pointer ispositioned on a plurality of markers, which are in a fixed relationshipto the coordinate systems of the fluoroscopic scanner. Thus, thesurgical tool coordinate system and the fluoroscopic scanner coordinatesystem are readily aligned.

U.S. Pat. No. 6,782,287 to Grzeszczuk et al, entitled “Method andApparatus for Tracking a Medical Instrument Based on Image Registration”is directed to an apparatus, method and system for tracking a medicalinstrument, as it is moved in an operating space, by constructing acomposite, 3-D rendition of at least a part of the operating space basedon an algorithm that registers pre-operative 3-D diagnostic scans of theoperating space with real-time, stereo x-ray or radiograph images of theoperating space. An x-ray image intensifier, mounted on a C-arm, and thesurgical instrument are equipped with emitters defining the localcoordinate systems of each of them. The emitters may be LED markerswhich communicate with a tracking device or position sensor. Theposition sensor tracks these components within an operating spaceenabling the coordinate transformations between the various localcoordinate systems. Image data acquired by the x-ray camera is used toregister a pre-operative CT data set to a reference frame of a patientby taking at least two protocoled fluoroscopic views of the operatingspace, including a patient target site. These images are then used tocompute the C-arm-to-CT registration. With the surgical tool beingvisible in at least two fluoroscopic views, the tool is thenback-projected into the reference frame of the CT data set. The positionand orientation of the tool can then be visualized with respect to a 3Dimage model of the region of interest. The surgical tool can also betracked externally using the tracking device.

U.S. Pat. No. 6,246,898 to Vesely et al. entitled “Method for CarryingOut a Medical Procedure Using a Three-Dimensional Tracking and ImagingSystem”, is directed to a system including a 3D tracking module, animaging modality, a registration module, an instrument (e.g., catheter),reference transducers and mobile transducers. The transducers may beultrasonic or electromagnetic transducers. The mobile transducers arecoupled with the instrument and with the 3D tracking module. Theregistration module is coupled with the 3D tracking module and with theimaging modality. The 3D tracking module transforms the measurements ofthe transducers into XYZ coordinates relative to a reference axis,indicating the position of the instrument. A 3D image, representing theposition, size and shape of the instrument, based on the 3D coordinates,is constructed. The imaging modality acquires 2D, 3D or 4D image datasets from an imaging source (e.g., MRI, CT, US). The registration moduleregisters the position of the instrument with the spatial coordinates ofthe image data set by registering features in the image, such as thereference transducers, with their position in the measuring coordinatesystem (i.e., 3D tracking module coordinate system).

U.S Patent application publication 2005/0182319 to Glossop entitled“Method and Apparatus for Registration, Verification, and Referencing ofInternal Organs”, is directed to a method for registering imageinformation of an anatomical region (image space) with positioninformation of a path within the anatomical region (patient space). Oneor more images of the anatomical region, are obtained (e.g., CT, PET,MRI). A three dimensional model of the anatomical region is constructed.The position information of the path within the anatomical region isobtained by inserting a registration device into a conduit, while atracking device simultaneously samples the coordinates of the positionindicating element coupled to the registration device. A threedimensional path (“centerline”) of the registration device, in theanatomical region, is determined. The registration device includes atleast one position indicating element (e.g., a coil that detects amagnetic field that is emitted by an electromagnetic tracking device).The image coordinate system is registered with the coordinate system ofthe tracking device, using the 3D image model and the 3D path of theregistration device. Thus, it is possible to represent on the image, agraphical representation of an instrument, equipped with a positionindicating element. However, in the method directed to by Glossop, thereis no guarantee that the three dimensional path, obtained by thetracking device, is indeed the path of the center of the conduit. It maybe that the tracking device traced a path close to the edges of theconduit or a sinusoidal path within the conduit. Therefore, theregistration between the image coordinate system, with the coordinatesystem of the tracking device, may be rendered inaccurate.

U.S. Patent Application Publication 2006/0262970, to Boese et al,entitled “Method and Device for Registering 2D Projection ImagesRelative to a 3D Image Data Record” directs to a method for registering2D projection images of an object relative to a 3D image data record ofthe same object. In the method to Boese et al, a pre-operative 3D datais recorded and a 3D feature (e.g., a model of a vessel tree) isextracted. The same 3D feature is recorded in at least two 2Dfluoroscopy images from different C-arm angulations). A 3D symbolicreconstruction of the feature is determined from the two 2D fluoroscopyimages. The coordinate systems of the 2D images and the 3D data areregistered according to the reconstructed 2D feature from the 2D imagesand the extracted 3D feature from the 3D data.

SUMMARY OF THE PRESENT DISCLOSED TECHNIQUE

It is an object of the disclosed technique to provide a novel method andsystem for registering a three dimensional (3D) pre-acquired imagecoordinates system with a Medical Positioning System coordinate systemand with a two dimensional (2D) image coordinate system.

In accordance with the disclosed technique, there is thus provided amethod for registering a 3D pre-acquired image coordinates system with aMPS coordinate system and with a 2D image coordinate system. The methodcomprises the procedure of acquiring at least one 2D image of a volumeof interest, acquiring a plurality of MPS points, within the at leastone tubular organ, extracting a 3D image model of the at least onetubular organ, estimating a volumetric model of the at least one tubularorgan and the 3D coordinate system with the MPS coordinate system andwith the 2D coordinate system. The volume of interest includes at leastone tubular organ. The 2D image is associated with the 2D coordinatesystem. The MPS points are associated with the MPS coordinate system.The MPS coordinate system is registered with the 2D coordinate system.The 3D image model is extracted form a pre-acquired 3D image of thevolume of interest. The 3D image is associated with the 3D coordinatesystem. The volumetric model is estimated from the 2D image and theacquired MPS points. The 3D coordinate system is registered with the MPScoordinate system and with the 2D coordinate system by matching theextracted 3D image model and the estimated volumetric model of thetubular organ.

In accordance with another aspect of the disclosed technique, there isthus provided a system for registering a three dimensional (3D)pre-acquired image coordinates system with a Medical Positioning System(MPS) coordinate system and with a two dimensional (2D) image coordinatesystem. The system includes a medical imaging for acquiring at least one2D image of a volume of interest, and a 3D medical images database forstoring pre-acquired 3D images of the volume of interest. The 2D imageis associated with the 2D coordinate system. The pre-acquired 3D imagesare associated with the 3D coordinate systems. The volume of interestincludes at least one tubular organ. The system comprises an MPS and acoordinate system registration processor. The MPS is associated with theMPS coordinate system. The MPS coordinate system is registered with the2D coordinate system. The MPS includes MPS transmitters and an MPSsensor for acquiring a plurality of MPS points within the at least onetubular organ. The coordinate systems registration processor is coupledthe MPS, with the medical imaging system and with the 3D medical imagesdatabase. The coordinate systems registration processor extracts a 3Dimage model of the tubular organ estimates a volumetric model of thetubular organ and registers the 3D coordinate system with the MPScoordinate system and with the 2D coordinate. The coordinate systemsregistration processor extracts the 3D image model form the pre-acquired3D image. The coordinate systems registration processor estimates thevolumetric model according to the 2D image and the acquired MPS pointsand registers the 3D coordinate system with the MPS coordinate systemand with the 2D coordinate by matching the extracted 3D image model andthe estimated volumetric model of the tubular organ.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed technique will be understood and appreciated more fullyfrom the following detailed description taken in conjunction with thedrawings in which:

FIG. 1A is a schematic illustration of a 3D pre-acquired image 100associated with a 3D coordinate system 104 in accordance with thedisclosed technique;

FIG. 1B is a schematic illustration of a 3D image model 106, of tubularorgan 102, extracted from 3D pre-acquired image 100 (FIG. 1A);

FIG. 2A is a schematic illustration of a trace 122 of a medical device(not shown) in accordance with the disclosed technique;

FIG. 2B is a schematic illustration of a 2D image 112 of the volume ofinterest;

FIG. 2C is a schematic illustration of estimated volumetric model 124determined from trace 122 (FIG. 2A) and 2D representation 114 (FIG. 2B)of the tubular organ;

FIG. 3 is a schematic illustration of a registration process inaccordance with the disclosed technique;

FIG. 4 is a schematic illustration of a system for registering a 3Dcoordinate system with an MPS coordinate system and with a 2D coordinatesystem, constructed and operative in accordance with another embodimentof the disclosed technique;

FIG. 5 is a schematic illustration of a method for registering a 3Dcoordinate system (e.g., of a pre-acquired volumetric image) with a 3DMPS coordinate system and with a 2D coordinate system (e.g. of areal-time image), operative in accordance with a further embodiment ofthe disclosed technique; and

FIG. 6A and 6B are schematic illustrations of three 2D images, oftubular organ in the body of a patient, acquired at three differentactivity states of the organ, and the MPS points acquired during thesethree different activity states, in accordance with another embodimentof the disclosed technique.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosed technique overcomes the disadvantages of the prior art byproviding a method and a system for registering a coordinate systemassociated with a three dimensional (3D) pre-acquired medical image witha 3D coordinate system associated with an MPS and with a 2D coordinatesystem associated with a 2D image. The system, according to thedisclosed technique, pre-acquires a 3D image of the volume of interest,and extracts a 3D image model of at least one tubular organ, within thevolume of interest, from that 3D image (e.g., the coronary vessel of theheart). The system further obtains an estimated volumetric model of thesame tubular organ. The system obtains this estimated volumetric model,using a trace of a medical device (i.e., a set of locations representingthe trajectory of the medical device), which is inserted into thetubular organ, and at least one 2D image of that same organ. The medicaldevice is fitted with an MPS sensor. The system uses these models toregister the above mentioned coordinate systems, thus achievingregistration with a higher degree of accuracy.

The coordinate system associated with the 3D pre-acquired image will bereferred to herein as 3D coordinate system. The coordinate systemassociated with the 2D image will be referred to herein as 2D coordinatesystem. The coordinate system associated with the MPS will be referredto herein as MPS coordinate system. It is noted that the MPS coordinatesystem is a 3D coordinate system. The term “registration” refers tofinding a transformation associating the coordinates of each point inone coordinate system to the coordinates of the same point in anothercoordinate system. The terms “trace” and “centerline”, both refer hereinto a set of locations representing the trajectory of the medical device.

A 3D pre-acquired image (e.g., CT, MRI, PET, 3D Ultra Sound) of a volumeof interest can serve as a 3D reference road-map for navigating aminimal invasive medical device, such as a catheter, in that volume.Superimposing a minimal invasive medical device, operative in an MPScoordinate system and fitted with an MPS sensor, on the 3D pre-acquiredimage, requires registering the 3D coordinate system with the MPScoordinate system.

To achieve the registration, prior to a medical procedure, the systemaccording to the disclosed technique, processes (e.g., segments) the 3Dpre-acquired image and extracts a 3D model of a tubular organ. Thetubular organ is situated within the imaged volume of interest. During amedical procedure (e.g., minimal invasive procedure), the medical staffinserts a medical device, fitted with an MPS sensor, into the tubularorgan. An MPS acquires a plurality of MPS points (i.e., a plurality oflocations of the MPS sensor within and along the tubular organ), anddetermines a 3D MPS trace of the shape of the same tubular organ. TheseMPS points are represented by coordinates in the MPS coordinate system.

When the medical staff inserts the medical device fitted with an MPSsensor into the tubular organ, the system obtains a 2D real-time image(e.g., X-ray, 2D Ultra Sound) of that organ. The MPS coordinate systemis registered with the 2D coordinate system (e.g., by mechanicallycoupling the MPS transmitters to the imager). Using the 3D MPS trace andat least one 2D image, the system estimates a volumetric model of thetubular organ, and registers the MPS coordinate system and the 3Dcoordinate system by matching the extracted image model with theestimated volumetric model. The system achieves this registration with ahigh degree of accuracy, (i.e., since a volumetric model represents thetubular organ with a high degree of accuracy, than a simple trace of thetrajectory of the MPS sensor within the tubular organ). Since the 2Dcoordinate system is registered with the MPS coordinate system, and theMPS coordinate system is registered with the 3D coordinate system, the2D coordinate system is also registered with the 3D coordinate system.

During the medical procedure, the position and orientation of a patientmight change. Consequently, the 2D real-time representation of thevolume of interest may also change. These changes may affect theregistration between the 3D coordinate system and the 2D coordinatesystem. Therefore, an MPS reference sensor, placed on the patient duringthe medical procedure, is operative to detect these changes in thepatient position and orientation. The information about these changesmay be used either for triggering a registration process or as input forsuch a registration process.

Reference is now made to FIG. 1A and to FIG. 1B. FIG. 1A is a schematicillustration of a 3D pre-acquired image 100 associated with a 3Dcoordinate system 104 in accordance with the disclosed technique. Image100 is a 3D image of a volume of interest which includes tubular organ102. FIG. 1 B is a schematic illustration of a 3D image model 106, oftubular organ 102, extracted from 3D pre-acquired image 100 (FIG. 1A).Extracted image model 106 is also associated with 3D coordinate system104.

Reference is now made to FIGS. 2A, 2B and 2C. FIG. 2A is a schematicillustration of a trace 122 of a medical device (not shown) inaccordance with the disclosed technique. Trace 122 is constructed from aplurality of MPS points, such as MPS point 120, representing thelocations of the MPS sensor, fitted on the medical device, acquired whenthe medical device moves along the tubular organ (i.e., pushed forwardor pulled back). These points are represented as coordinates in MPScoordinate system 118. FIG. 2B is a schematic illustration of a 2D image112 of the volume of interest. 2D image 112 includes a 2D representation114 of the tubular organ, and the trajectory 116 of the medical deviceinside this tubular organ. 2D image 112 is associated with 2D coordinatesystem 110. When the system according to the disclosed technique, usesan X-ray imager to obtain 2D image 112, it is desirable to inject thetubular organ with a dye to increase the apparentness of 2Drepresentation 114 of the tubular organ in image 112. Since MPScoordinate system 118 is registered with 2D coordinate system 110, eachof the MPS points, such as MPS point 120, has a corresponding point in2D coordinate system 110. Using image processing techniques, such assegmentation or edge detection, the system determines the width of 2Drepresentation 114 of the tubular organ for each MPS point. The systemuses this width, together with trace 122 of the medical device (i.e.,not necessarily the centerline of the tubular organ), to determine anestimated volumetric model of the tubular organ. For example, the widthof 2D representation 114 of the tubular organ, at each MPS point,determines the diameter of a circle encircling that point. FIG. 2C is aschematic illustration of estimated volumetric model 124 determined fromtrace 122 (FIG. 2A) and 2D representation 114 (FIG. 2B) of the tubularorgan. Estimated volumetric model 124 is associated with MPS coordinatesystem 118 and with 2D coordinate system 110.

Reference is now made to FIG. 3 which is a schematic illustration of aregistration process in accordance with the disclosed technique. In FIG.3, the system registers MPS coordinate system 118 with 3D coordinatesystem 104, for example, by matching extracted 3D model 106 withestimated volumetric model 124. Consequent to this registration, 2Dcoordinate system 110 is also registered with coordinate system 104.Thus, each point, in each one of coordinate systems 110, 118 and 104,has a corresponding point in each of the other coordinate systems. Thisregistration, between coordinate systems 110, 118 and 104, enablessuperimposing MPS points of interest, at their respective locations onthe 3D image. For example, the 3D pre-acquired image may now serve, forexample, as a roadmap for the medical staff, during medical procedures(e.g., treating structural heart disease, deployment of percutaneousvalves, ablation, mapping, drug delivery, ICD/CRT lead placement,deploying a stent and other PCI procedures, surgery, biopsy). On this 3Dreference roadmap, the system superimposes the 3D trace of the medicaldevice within the tubular organ. This registration further enablessuperimposing points of interest included in the 3D image, at theirrespective location on the 2D image. As a further example, the 3D imagemodel of the tubular organ may be projected on the 2D image. Thus, theprojected 3D image may serve as a virtual dye, instead of injecting afluoroscopic dye to the tubular organ prior to obtaining the 2D image.

Reference is now made to FIG. 4, which is a schematic illustration of asystem, generally referenced 150, for registering a 3D coordinate systemwith an MPS coordinate system and with a 2D coordinate system,constructed and operative in accordance with another embodiment of thedisclosed technique. System 150 includes medical imaging system 168, aMedical Positioning System (MPS) 174, a 3D medical images database 176,a registration processor 178, a catheter 156, a display unit 172 and atable 154. Medical imaging system 168 includes an imaging radiationtransmitter 170 and an imaging radiation detector 166. Medicalpositioning system 174 includes MPS transmitters 160, 162 and 164,attached to imaging radiation detector 166, patient reference positionsensor 180 and MPS sensor 158.

Display unit 172 is coupled with coordinate systems registrationprocessor 178. Coordinate systems registration processor 178 is furthercoupled with 3D medical images database 176, with MPS 174 and withimaging radiation detector 166. MPS sensor 158 is fitted on the distalend catheter 156. MPS transmitters 160, 162 and 164 are mechanicallycoupled with imaging radiation detector 166.

The 3D pre-acquired medical images, stored in 3D medical images database176, are associated with a 3D coordinate system. The images acquired bymedical imaging system 168 are associated with a 2D coordinate system.MPS 174 is associated with an MPS coordinate system. As mentioned above,since MPS transmitters 160, 162 and 164 are mechanically coupled toimaging radiation detector 166, the MPS coordinate system is registeredwith the 2D coordinate system. However, when MPS transmitters 160, 162and 164 are not mechanically coupled with imaging radiation detector166, the MPS coordinate system may be registered with the 2D coordinatesystem by placing an MPS sensor, at the 2D image space as a fiducialmark, at pre-determined positions and acquiring a 2D image of the MPSsensor at these locations. MPS 174 determines the location of the MPSsensor in the MPS coordinate system. Registration processor 178determines the position of the MPS sensor in a plurality of 2D imagesand registers the 2D coordinate system with the MPS coordinate system.

A member of the medical staff inserts catheter 156 in to a patient 152,lying on table 154 and subjected to a treatment, and navigates thecatheter, inside a tubular organ toward a volume of interest (e.g., thecardiovascular system). MPS transmitters 160, 162 and 164 transmitmagnetic fields which are mutually orthogonal, corresponding to axes ofthe MPS coordinate system. MPS sensor 158 detects the magnetic fieldsgenerated by MPS transmitters 160, 162 and 164. The detected signals arerelated to the positions of distal end 158, in the MPS coordinatesystem, for example, by the Biot Savart law. Medical positioning system174 obtains a trace of catheter 156 within a tubular organ, situatedwithin the volume of interest. MPS 174 provides this trace toregistration processor 178.

Imaging radiation transmitter 170 transmits radiation that passesthrough patient 152. This radiation, detected by imaging radiationdetector 166, is a 2D projection of the anatomy of a volume of interestof patient 152. Imaging radiation detector 166 provides the 2D image tocoordinate systems registration processor 178.

Using the MPS trace of catheter 156 and the 2D image, registrationprocessor 178 constructs an estimated volumetric model of the tubularorgan. 3D images database 176 provides coordinate systems registrationprocessor 178 a 3D pre-acquired image of the same volume of interest ofpatient 156. Registration processor 178 extracts a 3D image model of thetubular organ. The two models are, for example, 3D triangulated meshrepresentations of the tubular organ. Registration processor 178registers the 3D coordinate system with the MPS coordinate system, forexample, by matching the two models.

Reference is now made to FIG. 5, which is a schematic illustration of amethod for registering a 3D coordinate system (e.g., of a pre-acquiredvolumetric image) with a 3D MPS coordinate system and with a 2Dcoordinate system (e.g. of a real-time image), operative in accordancewith a further embodiment of the disclosed technique. In procedure 200,a volume of interest is selected. The volume of interest includes atleast one tubular organ. After procedure 200 the method proceeds toprocedures 202, 206 and 208.

In procedure 202, a 3D image of the selected volume of interest ispre-acquired. The 3D image is associated with a 3D coordinate system.This 3D image may be, for example, an MRI image, a PET image, a 3Dreconstructed Ultrasound image, and the like. The pre-acquired 3D imageis stored in a database. With reference to FIG. 1A, image 100 is anexemplary 3D image. With reference to FIG. 4, 3D medical image database176 stores the 3D pre-acquired image. After procedure 202 the methodproceeds to procedure 210.

In procedure 204, an MPS coordinate system is registered with a 2Dcoordinate system. This registration is achieved, for example, bymechanically coupling the MPS transmitters to the imaging system.Alternatively, for example, an MPS sensor is placed in the 2D imagespace as a fiducial mark, at pre-determined positions and 2D images ofthe MPS sensor are acquired at these locations. MPS 174 determines thelocation of the MP sensor in the MPS coordinate system. Registrationprocessor 178 determines positions of MPS sensor in the 2D image andregisters the 2D coordinate system with the MPS coordinate system.Consequent to this registration, each point in the MPS coordinate systemhas a corresponding point in the 2D coordinate system. With reference toFIG. 4, the MPS coordinate system is registered with the 2D coordinatesystem by mechanically coupling MPS transmitters 160, 162 and 164 toimaging radiation detector 166. After procedure 204 the method proceedsto procedure 208.

In procedure 206, at least one 2D real-time medical image is acquired.This 2D real-time medical image is, for example, an X-ray image, of aprojection of a volume of interest in a body of a patient. The 2Dreal-time image is acquired, for example, during a medical procedureinvolving the use of a medical device, such as a catheter. Withreference to FIG. 4, 2D medical imaging system 168 acquires at least one2D real-time medical image. With reference to FIG. 2B, image 112 is, forexample, a 2D real-time medical image. After procedure 206 the methodproceeds to procedure 212.

In procedure 208, a plurality of MPS points, within at least one tubularorgan, are acquired in real-time, the tubular organ being within theselected volume of interest. These MPS points are associated with theMPS coordinate system. The MPS points are acquired with a catheter,fitted with an MPS sensor, inserted into the tubular organ. The MPSpoints are acquired during the insertion of the catheter or during amanual or automatic pullback of the catheter. These MPS points form atrace of the trajectory of the catheter within the tubular organ. Withreference to FIG. 4, MPS 174 acquires a plurality of MPS points, withinthe tubular organ. MPS 174 acquires these points with MPS sensor 158fitted on catheter 156. With reference to FIG. 2A, trace 122 is formedfrom a plurality of MPS points such as point 120. After procedure 208the method proceeds to procedure 212.

In procedure 210, at least one 3D image model of the at least onetubular organ is extracted from the pre-acquired 3D image. This 3D imagemodel is, for example, a 3D triangulated mesh representation of thetubular organ. The 3D image model of the tubular organ is extracted forexample by segmenting the 3D pre-acquired image. With reference to FIGS.1A, 1B and 4, registration processor 178 (FIG. 4) extracts a 3D imagemodel, such as 3D image model 106 (FIG. 1B), from a 3D pre-acquiredimage, such as 3D pre-acquired image 100 (FIG. 1A). The 3D pre-acquiredimage is stored in a 3D medical image database 176 (FIG. 4). Afterprocedure 210 the method proceeds to procedure 214.

In procedure 212, a volumetric model of the at least one tubular organis estimated according to the at least one 2D image and the acquired MPSpoints. This volumetric model of the tubular organ is estimated bydetecting the border points of the tubular organ, for each point in theat least one 2D image, corresponding to an MPS point. These borderpoints determine the constraints of a closed curve generated around eachpoint on the at least on 2D image (e.g., the circumference of the closedcurve must include these border points). In the case wherein one 2Dimage, of the tubular organ, was acquired from one perspective, theclosed curve is a circle. The diameter of that circle is the distancebetween the detected 2D borders of the tubular organ. When, for example,two 2D images of the tubular organ, were acquired from two differentperspectives, the refined contour will have the shape of an ellipse. Inthe case wherein more than two 2D images were acquired, the shape of theclosed curve changes accordingly. The estimated volumetric model is alsorepresented as a 3D triangulated mesh. It is noted that the MPS pointneed not be at the center of the closed curve. With reference to FIGS.2A, 2B, 2C and 4, registration processor 178 estimates a volumetricmodel of the tubular organ such as volumetric model 124 (FIG. 2C).Registration processor 178 estimates this volumetric model form at leastone 2D image such as 2D image 112 (FIG. 2B) and a plurality of MPSpoints such as MPS point 120 (FIG. 2A). After procedure 212 the methodproceeds to procedure 214.

In procedure 214, the 3D coordinate system is registered with the MPScoordinate system and with the 2D coordinate system, by matching theextracted 3D image model with the estimated volumetric model of thetubular organ. This registration is performed, for example, by matchingthe 3D representations of the two models, thus achieving registrationwith a high degree of accuracy (i.e., since a volumetric modelrepresents the tubular organ with a higher degree of accuracy, than asimple trace of the trajectory of the MPS sensor within the tubularorgan). With reference to FIG. 4, registration processor 178 registersthe MPS coordinate system with the 3D coordinate system and with the 2Dcoordinate system.

It is noted that the system and the method described in conjunction withFIG. 4 and FIG. 5, relate to the case wherein the 3D pre-acquired image,the MPS model and the 2D image are static. However, the disclosedtechnique is readily extended to the case where the 3D and 2D imageschange with time, for example, as a result of respiration and cardiacmotion due to the cyclic motion of the heart and lungs. A cardiac cycleis defined as the time between two subsequent heart contractions, andthe respiratory cycle is defined as the time between two subsequent lungcontractions. It is noted that a time changing 3D or 2D image, iscomposed of a plurality of static 3D or 2D images respectively, eachvisually representing the organ at a different state. Furthermore, it isnoted that for each static 3D or 2D image there is an organ activitystate (i.e., a point within the cardiac or respiratory cycles)associated therewith.

As mentioned above, during acquisition of the MPS points, the inspectedtubular organ may move (e.g., due to the cardiac and respiratorymotion). This motion affects the MPS sensor readings (e.g., position andorientation). Therefore, while acquiring MPS point readings, and 3D and2D time changing images, the system simultaneously acquires organ timingsignals of the organ (e.g., the heart, the lungs). These organ timingsignals represent the activity states in the cycle of the organ.Accordingly, each MPS point, and each of the static 3D and 2D images,(i.e., the static 3D and 2D images composing the time changing 3D and 2Dimages), is associated with a respective organ timing signal phase(i.e., an activity state of the organ), for example, anelectrocardiogram (ECG) signal. Consequently, each MPS point isassociated with a respective static 3D and 2D image, according to therespective organ timing signals thereof. A plurality of MPS points,acquired during the same activity state, define a unique 3D trajectoryof the MPS sensor associated with the respective activity state. Thus,the unique 3D trajectory, of the MPS sensor, is also defined for eachstatic 3D and 2D image.

Reference is now made to FIG. 6A and 6B, which are schematicillustrations of three 2D images, of a tubular organ in the body of apatient, acquired at three different activity states of the organ, andthe MPS points acquired during these three different activity states, inaccordance with another embodiment of the disclosed technique. The firstimage of the organ, designated 250 ₁, was acquired at a first activitystate T₁. The second image of the organ, designated 250 ₂, was acquiredat a second activity state T₂. The third image of the organ, designated250 ₃, was acquired at a third activity state T₃. During activity stateT₁, MPS points 252, 258 and 264 were acquired. During activity state T₂,MPS points 254, 260 and 266 were acquired. During activity state T₃, MPSpoints 256, 262 and 268 were acquired. Thus, referring to FIG. 6B,centerline 270 is the projection of the catheter 3D trajectory on image250 ₁. Centerline 272 is the projection of the catheter 3D trajectory onimage 250 ₂. Centerline 274 is the projection of the catheter 3Dtrajectory on image 250 ₃. Consequent to associating the MPS points, the2D images and the 3D images with a respective organ timing signal, thesystem according to the disclosed technique can superimpose the unique3D trajectory on the respective static 3D or 2D image (i.e., the imageassociated with the same activity state). Furthermore, only atrajectory, a 2D image and a 3D image, associated with a single organtiming signal reading of interest can be considered. For example,referring back to

FIG. 6B, only image 250 ₃, with centerline 274 is considered. A patientreference sensor, such as patient reference sensor 180 (FIG. 4),compensates respiration artifacts as well as patient movements duringthe acquisition of the 3D, 2D images or the organ timing signal. It willbe appreciated by persons skilled in the art that the disclosedtechnique is not limited to what has been particularly shown anddescribed hereinabove. Rather the scope of the disclosed technique isdefined only by the claims, which follow. cm 1.-25. (canceled)

26. Method for registering a Medical Positioning System (MPS) coordinatesystem with a two dimensional (2D) coordinate system, the methodcomprising: acquiring at least one image of a volume of interest, thevolume of interest within the body of a patient, the image beingassociated with the 2D coordinate system; acquiring a plurality of MPSpoints, within the body of the patient, the plurality of MPS pointsbeing associated with the MPS coordinate system, wherein each of theplurality of MPS points and each of the at least one image, are allassociated with an organ timing signal activity state; and estimating avolumetric model within the body of the patient according to acquiredMPS points.
 27. The method according to claim 26, further comprising theprocedure of registering the 2D coordinate system with the MPScoordinate system, prior to acquiring the at least one image and the MPSpoints.
 28. The method according to claim 27, wherein the procedure ofregistering is performed between the MPS and the at least one image, allassociated with the same organ timing signal activity state.
 29. Themethod according to claim 26, wherein the organ timing signal tracks arespiratory cycle.
 30. The method according to claim 26, wherein theorgan timing signal represents an activity state in a cycle of an organ.31. The method according to claim 26, wherein the organ timing signal isan electrocardiogram signal.
 32. The method according to claim 26,further comprising registering the MPS coordinate system with the MPScoordinate system and wherein the MPS coordinate system and the 2Dcoordinate system are registered with each other by placing a fiducialmark at a predetermined position and acquiring an image of the fiducialmark at the predetermined position.
 33. The method according to claim26, wherein each of the plurality of MPS points is associated with oneof the at least one image according to the organ timing signal activitystate.
 34. The method according to claim 26, further comprising defininga 3D trajectory of the MPS sensor.
 35. The method according to claim 34,wherein the 3D trajectory is defined using a plurality of MPS pointsacquired during the same activity state.
 36. The method according toclaim 34, wherein the 3D trajectory is defined for each of the at leastone image.
 37. System for registering a Medical Positioning System (MPS)coordinate system with a two dimensional (2D) coordinate system, thesystem including a medical imaging system for acquiring at least oneimage of a volume of interest within the body of a patient the at leastone image being associated with the 2D coordinate system, the systemcomprising: an MPS associated with the MPS coordinate system, the MPScomprising: a plurality of MPS transmitters for generating anelectromagnetic field; and an MPS sensor, for acquiring a plurality ofMPS points within the volume of interest within the body, the MPS sensorbeing associated with the MPS coordinate system, the MPS coordinatesystem being registered with the 2D coordinate system, wherein each ofthe MPS points and the at least one image, are all associated with arespective organ timing signal activity state of an organ; and aprocessor coupled with one of the MPS and the medical imaging system,and configured to receive data from the other of the MPS and the medicalimaging system, the processor estimating a volumetric model within thebody of the patient according to acquired MPS points.
 38. The systemaccording to claim 37, wherein the organ timing signal represents anactivity state in a cycle of the organ.
 39. The system according toclaim 37, wherein the organ timing signal is respective of the heart ofthe patient.
 40. The system according to claim 37, wherein the organtiming signal is respective of the lungs of the patient.
 41. The systemaccording to claim 37, wherein the coordinate systems registrationprocessor registers the MPS coordinate system with the 2D coordinatesystem according to the MPS points and the at least one image associatedwith the same respective organ timing signal activity state.
 42. Thesystem according to claim 37, further comprising registering the MPScoordinate system with the MPS coordinate system and wherein the MPScoordinate system and the 2D coordinate system are registered with eachother by placing a fiducial mark at a predetermined position andacquiring an image of the fiducial mark at the predetermined position.43. The system according to claim 37, wherein the processor is furtherconfigured to define a 3D trajectory of the MPS sensor.
 44. The systemaccording to claim 43, wherein the 3D trajectory is defined using aplurality of MPS points acquired during the same activity state.
 45. Thesystem according to claim 43, wherein the 3D trajectory is defined foreach of the at least one image.